A radio communication system permits transmission of information between a transmitter and a receiver. A radio frequency (RF) channel permits transmission of information between the transmitter and the receiver. By combining the information with an RF electromagnetic wave of a particular frequency, i.e., modulating the information signal onto a carrier frequency, the resultant modulated information signal may be transmitted through free space to a receiver. Various modulation techniques (e.g., amplitude (AM), frequency (FM), phase, and composite modulation) are known to combine the information signal with an electromagnetic wave. Communication units, such as portable radios, mobile radios, and base stations, contain transmitters and/or receivers.
A linear AM transmitter does not have as much coverage area, i.e., the signal does not travel as far, as an FM transmitter at the same peak transmit power level because the average envelope size of an AM transmission varies below the maximum output level, whereas the average envelope size of an FM transmission is constant at the maximum output level. An FM transmitter, however, uses more energy to transmit at the same power level as an AM transmitter, and hence the FM transmitter will more quickly drain the battery of a portable transmitter.
Accordingly, there is a need for a transmitter which has the low power characteristic of an AM transmitter while retaining the advantage of coverage area of an FM transmitter.